U.S. Pat. Nos. 3,172,862, and 3,173,050 both granted in the 1965 teach the preparation of an organic electroluminescence (EL) device using conjugated materials generally having a fused aromatic ring. The efficiency and life of such organic EL devices was much lower than those obtained from inorganic systems. Therefore, research at that time was mainly focused on inorganic materials. The reason for the low luminance of early organic EL devices was the highly resistive EL medium, which prevented the efficient injection and transport of charge carriers into the light-emitting layer. Tang and VanSlyke (Tang and VanSlyke, Appl. Phys. Lett. 1987, 51, 913) solved this problem successfully in the late 1980s by using a structure made of two ultra thin layers: a hole transporting layer of an organic substance laminated on an organic emitting layer. This work revived the research into organic EL devices and resulted in the development of a new generation of light-emitting diodes based on organic dyes. Since then, much work has been done to further improve the efficiency, stability, color purity and so forth of such devices. One improvement was to dope a strong emitting material into a host material to form a guest-host system. Thus, in principle, an organic EL device with good efficiency and high stability, as well as desired color with proper chromaticity, could be obtained by doping different strongly emitting materials into a host material such as tri-(8-hydroxyquinolinato)aluminum (Alq3) to meet the requirement of the practical applications. As a general rule, the energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of a host material should be larger than that of the doped guest material to allow an efficient energy transfer from the host to guest.
Alq3 is one of the most widely used host materials in organic EL devices. Its use as a host material for a green-emitting device and a red-emitting device made of DCM derivatives is ought in U.S. Pat. Nos. 5,593,788 and 5,935,720 respectively. The guest material, or dopant, that emits at longer wavelengths, such as green, yellow and red region can be, for example, compounds with large fused homo-aromatic rings or intramolecular charge transfer (ICT) compounds with electron donating (D) groups and electron withdrawing groups (A) linked by a conjugated structure. Since the fused homo-aromatic compounds with a large conjugated structure, especially those that emit in red, are often oxidized easily by singlet oxygen in ambient conditions they are not suitable dopants unless the EL devices are used in the dark or without oxygen. Compared with the fused homo-aromatic rings compounds, ICT compounds have the following advantages:
(i) The emission wavelength can be easily tuned by changing substitutes to obtain different colors.
(ii) The molecular structure is relatively easy to modify for desired properties.
(iii) Their Stokes shifts are generally large to prevent efficient self-re-absorption, especially in the solid state.
(iv) They are chemically stable, not easily oxidized by singlet oxygen.